U.S. Pat. No. Re. 27,617 (Olsen) discloses a process of making a louvered light control film by skiving a billet of alternating layers of plastic having relatively low (clear) and relatively high (pigmented) optical densities. Upon skiving the billet, the pigmented layers provide light-collimating louver elements which, as illustrated in the patent, may extend orthogonally to the surface of the resulting louvered plastic film. U.S. Pat. No. 3,707,416 (Stevens) discloses a process whereby the light-collimating louver elements may be canted with respect to the surface of the louvered plastic film. U.S. Pat. No. 3,919,559 (Stevens) teaches a process for attaining a gradual change in the angle of cant of successive light-collimating louver elements.
Among uses for louvered plastic films are as lenses in goggles as shown in U.S. Pat. No. 3,791,722 (Ahlberg et al.) to be worn where high levels of illumination or glare are encountered. When used as a transparent covering for a back-lighted instrument panel, a louvered plastic film minimizes reflections, e.g., from being cast onto the windshield of an automobile. A louvered plastic film can also be used to give a black-and-white photographic negative the appearance of a positive made from the negative, as taught in U.S. Pat. No. 3,653,138 (Cooper).
U.S. Pat. No. 4,128,685 (Lowrey et al.) reports that billets from which louvered plastic films have been skived often are heated during the skiving, but that the heat "may tend to be absorbed preferentially by the opaque louver material so that the billet employed is softer along the louver lines than along the clear or transparent lines" (col. 1, lines 13-21). The patent teaches how to select materials for the billet so that the alternating clear and pigmented (opaque) layers have relatively uniform heat absorptive ability. The preferred material for the pigmented layers comprises self-crosslinking anionic acrylates, water-soluble polyazo direct dyes such as "Formanil Black G", and finely divided silica.
U.S. Pat. No. 3,922,440 (Wegwerth et al.) points out that because louvered plastic films "are thin sheet materials: (1) they are not by themselves capable of structurally withstanding extreme stresses and (2) they are subject to distortion from physical stress and temperatures" (col. 1, lines 19-22). Furthermore, the skiving by which the louvered plastic films are produced results in irregular surfaces. Accordingly, as in Example 1 of that patent, the louvered plastic film usually is laminated under pressure between two clear plastic films, e.g., cellulose acetate butyrate, the material usually used in making louvered plastic films. Typically, the louvered plastic film is skived to a thickness from 0.2 to 0.4 mm, and each of the outer clear plastic films has a thickness of about 0.15 to 0.3 mm.
Wegwerth's process of laminating louvered plastic films between two clear films requires an expensive press that is expensive to operate, in part from the need to distribute heat uniformly, and in part from the need to apply pressure with precision. Because the resulting laminates cannot be larger than the platens of the press in which they are laminated, the press must be sufficiently large and expensive to produce the largest required size.